1. Field of the Invention
The present invention relates to a device for press forming a sheet blank on a forming die of elastic material.
2. Description of the Related Art
Devices for press forming sheet blanks are disclosed in FR-A-2 641 215 and FR-A-2 641 217 and are of the type comprising a pad or cushion of an elastic material which is disposed in a retaining box and on which rests a lower blank holder, means for preforming the sheet blank constituted by an outer slide carrying an upper blank holder, and means for finally forming said sheet blank constituted by a central slide acting on a plunger.
The pad of elastic material may be a composite support composed of a substantially parallelepipedic mass based on a silicone elastomer of low Shore hardness which may be covered on its upper face and on the whole or a part of its lateral faces with a relatively thin covering of a stronger and harder material.
In the course of the preforming stage, the upper blank holder applied on the peripheral part of the sheet blank causes the flow of the mass of the elastic pad for deforming the central part of said sheet blank while imparting thereto, at the end of the preforming stage, a surface substantially equal in area to the surface of the part to be obtained.
The descent of the plunger for the final shaping of the sheet blank results in a rise in pressure in the pad and a difference of volume in said pad between the preforming stage and the shaping stage which must be compensated for so as to avoid an increase in the pressure in the pad.
Indeed, this pressure increase is incompatible with the mechanical resistance of the presses employed.
For this purpose, the device disclosed in FR-A-2 641 217 comprises means for absorbing the excess volume of the pad relative to the volume defined by the surface of the finished part to be obtained.
These absorbing means are movable in the course of the final forming of the part and this movement is regulated by a regulating means constituted for example by a multi-blade brake or a spring, or a cylinder device.
This known device permits absorbing the large, constant and defined differences of volume in the course of the first cycle of the press for a given geometry of the parts to be formed and at a given temperature.
However, this device has the drawback of failing to resolve the problem of a progressive and even a slight increase in the volume of the pad.
This increase may be due to variations in the physical properties of the material of the pad, such as for example a continuous heating of several tens of degrees generated by the deformation of the pad under the effect of the upper blank holder whose travel may represent 20% of the uncompressed height of the pad during the production of parts at a very high rate of operation of the press, i.e. higher than around ten cycles per minute.
These variations in the physical properties of the material of the pad imply successive regulations of the press which lowers the production rates.
There is known from the French patent application No. 92 06 502 in the name of the applicant, a device for press forming sheet stock on a forming die of an elastic material which comprises second means for compensating for the excess volume of the pad due to variations in the physical properties of the material of said pad in the course of the preforming and final forming of the part.
These second means for compensating for the excess volume of the pad comprise, on one hand, at least one slidable element disposed in a chamber opening onto the pad retaining box and adapted to cooperate with said pad and, on the other hand, an element for regulating the displacement of the slidable element interposed between said slidable element and the bottom of said chamber.
Although this device permits a fine compensation of the excess volume of the pad, it requires a precise regulation and does not fully satisfy the law of variation of the pressure of the pad due to the variation in the temperature as a function of the number of cycles of operation of the press. An adjustment remains essential in the course of the mass-production forming of sheet blanks.